Various methods of sterilization have been used in the past for the sterilization of different types of articles including disposable and reusable medical equipment, foods and food containers. Sterilization by steam or by dry heat has been extensively used in the past. Sterilization by heat, either wet or dry, is not useful to sterilize materials that are adversely effected by such heat or steam. Ethylene oxide gas has also been used but suffers from the drawback that it may leave toxic residues on the articles to be sterilized, which may have adverse effects, particularly on patients who come into contact with such articles. The extended aeration cycles required to remove residual ethylene oxide from some sterilized items also makes ethylene oxide sterilization excessively long.
The use of plasma to sterilize containers was suggested in U.S. Pat. No. 3,383,163. Plasma is an ionized body of gas which may be generated by the application of power from different sources. The ionized gas will contact microorganisms on the surface of the items to be sterilized and effectively destroy the microorganisms.
U.S. Pat. No. 3,851,436 discloses the use of radio frequency generators to produce such plasmas from inert gases such as argon, helium or xenon. U.S. Pat. No. 3,948,601 also discloses the use of a radio frequency generated plasma which ionizes argon, nitrogen, oxygen, helium or xenon. The processes set forth in the above-mentioned patent require the direct contact of the plasma on the surface of the product to be sterilized, which product is not packaged at the time of sterilization. The commercial sterilization procedures used to sterilize disposable medical goods generally require that the medical goods be packaged prior to sterilization because of the possibility of contamination by microorganisms if the products are packaged subsequent to sterilization.
U.S. Pat. No. 4,207,286 discloses a gas plasma sterilization system which uses glutaraldehyde as the gas which is used in a plasma sterilization system. The item to be sterilized is placed in an unsealed container or package and then subjected to the sterilization cycle. When the sterilization cycle is completed, the containers are sealed. The container must be opened during the sterilization cycle to allow the gas to flow into the interior of the package or container to allow contact of the gas with any microorganisms which may be on the surface of the item to be sterilized.
U.S. Pat. No. 4,321,232 discloses a plasma sterilization system in which the item to be sterilized is placed in a package made from a porous material. The gas used in the process is oxygen, and it is indicated that sterilization can be accomplished through the porous packaging within 60 minutes.
U.S. Pat. No. 4,348,357 discloses a plasma sterilization procedure using oxygen, nitrogen, helium, argon or freon as the gas. The pressure is pulsed, that is, the pressure within the container is alternately increased or decreased in a cyclic fashion. In addition, the plasma may be de-energized during the pressure fall portion of the pressure cycle to reduce the heating effect on the article to be sterilized.
Japanese Application Disclosure No. 103460-1983 discloses a plasma sterilization process in which the gas consists of nitrous oxide or a mixture of nitrous oxide with another gas such as oxygen, helium or argon. It is stated that the process can be used to sterilize through packaging and, particularly, packaging which is made from polyethylenetrifluoride or polyethylenetetrafluoride resins or paper coated with these materials.
Japanese Application Disclosure No. 162276-1983 discloses the sterilization of foods using nitrogen oxide gas or mixtures of nitrogen oxide gas and ozone in a plasma.
All of these prior plasma sterilization systems have not been put into wide commercial use because of the limitations on the time required to effect sterilization, the temperature obtained in the sterilization process or the particular requirements of some of the processes that would require post-sterilization packaging.
Hydrogen peroxide has been known to have bactericidal properties and has been used in solutions to kill bacteria on various surfaces. U.S. Pat. No. 4,437,567 discloses the use of aqueous hydrogen peroxide solutions at low concentrations, i.e., 0.01% to 0.10% by weight, to sterilize packaged products for medical or surgical use. At room temperature sterilization requires at least 15 days. At higher temperatures sterilization can be accomplished in approximately one day.
U.S. Pat. Nos. 4,169,123; 4,169,124 and 4,230,663 disclose the use of hydrogen peroxide in the gas phase at temperatures below 80.degree. C. and concentrations of 0.10 to 75 mg H.sub.2 O.sub.2 vapor/L for sterilization and disinfection. Depending upon concentration and temperature, sterilization times are reported to vary from 30 minutes to four hours.
The use of ultraviolet radiation with hydrogen peroxide for improved antimicrobial activity has been disclosed in U.S. Pat. Nos. 4,366,125 and 4,289,728. The lack of penetration by UV radiation below the surface of the object to be sterilized limits the application of this effect to clear solutions or surfaces that can be directly exposed to the radiation. Objects in an opaque package, or objects in a clear package that absorbs UV light could not be sterilized.
Food packaging materials sterilized with hydrogen peroxide contain hydrogen peroxide residuals that must be removed from the materials prior to use. U.S. Pat. No. 4,368,081 discloses the use of antioxidants or reducing agents such as L-ascorbic acid to remove residual hydrogen peroxide from a sterilized food package.
The combination of hydrogen peroxide and plasma has heretofore not been used for sterilization.